This study cloned the hemoglobin α1 from the marine teleost, the half-smooth tongue sole ( Cynoglossus semilaevis
), and then examined its expression under hypoxia exposure. The full-length of CsHb-α1 (594 bp) cDNA contains an open reading frame encoding 144 amino acids. Sequence analysis shows that the predicted CsHb-α1 amino acids shares high identities with that of other species. Real-time PCR showed that CsHb-α1 was highly expressed in the heart, liver, spleen, kidney and blood. Five to 120 min esposure and long-term (36 h) exposure to hypoxia (1.0 mg/L) significantly increased CsHb-α1 mRNA expression in most tissues compared to those fish held in normoxic conditions (dissolved oxygen (DO): 6.2 mg/L). These results suggested that the up-regulation of Hb-α1 is an important component for adaptation of half-smooth tongue sole to short-term hypoxia.